Microtomes are designed for precise and thin cutting of preparations for subsequent microscopic examination. For that purpose, a relative motion takes place between the cutting knife and the preparation. Once a cut has been made, the preparation and the knife must be advanced again with respect to one another by an amount equal to the desired cut thickness. This is generally accomplished with a spindle drive system and a movable nut. Rotation of the spindle causes the nut to move, and the desired advance motion takes place between the knife and the preparation or the specimen holder carrying the preparation.
A spindle drive system of this kind is depicted and described in DE 37 27 975 A1. This spindle drive system is characterized in that the usual play between the nut and spindle is minimized by way of a nut, in two parts braced against one another, that runs on the spindle. Largely zero-clearance advance, and thus precise cutting of the specimens, is thus possible.
These drive systems have proven successful for microtomes under normal conditions, but are insufficiently usable in cryostat microtomes. Because of the large temperature differences (from ambient temperature to −60° C.) during operation of a cryostat microtome, and the differing temperature coefficients of the materials used, a clearance that is in fact undesirable must be permitted in the connection between spindle and nut. Otherwise the risk exists that the drive system will jam.
A device that solves this problem is known from DE 36 15 714 A1. This document describes a motorized advance drive system for a specimen head of a cryostat microtome in which the specimen head is mounted slidingly via a sleeve, and is connected nonpositively to a motorized micrometer. As a result of the nonpositive connection, the preparation arm can be both advanced onto the cutting knife and removed again. With this apparatus, however, provision is also made to encapsulate the entire motorized micrometer drive system in a separate housing, and to heat that housing. Encapsulating and heating a microtome in a cryostat of course requires a great deal of complexity. In addition, a constant temperature setting for the cold portion of the cryostat is not possible with a design of this kind.